My friend has a CBR125R which is a 2008 model I believe (Australia) who had his bike fail on him randomly one day, he wasn't particularly sure what caused the problem, upon giving it in for servicing they informed it was the main bearing that failed, and they were going to charge some $3000 for fixing it, and the labour for putting it back together.

My friend said no, because that's how much he bought it for and it's been in pieces in different boxes for a year or so, until now...

I'm not very knowledgeable about engines, but I've seen work done on them before and have some idea of what to do, however I can't seem to find much wrong with the engine or any other components, the bearings look pretty solid to me, and they are rotating fine without any hindrance. I took some pictures of the parts in question before we start assembling it because we aren't sure of some aspects:



For this one, near my hand you can see some kind of fracture/stress marks in the crevice, I'm not sure what caused that, although it's a bit sharpish and not internal cracks.

 

Define rotating without hindrance do they grind do they feel gritty at all. They should be smooth as butter. The marks may be just casting marks.

 

They are very smooth, there's no noise and all the bearings seems to be well-oiled, if I didn't know any better I'd say they were new.

 

Well then I say put it together and check the clearances as you go but for the most part a bearing is not gonna stop an eng from running unless it just seizes up completely.

 

What was the problem it was having that prompted you to take it in?
Are you sure they didn't say "rod bearings?"

 

I'm not too sure because it's my friend's bike. I spoke to him and he said all he remembers is, on the day the engine was making an unusual noise but he can't remember what it sounded like exactly, and then later on in the day, the revs just started dropping slowly and it suddenly stalled and he couldn't get it started again.

I'm not too sure what you mean by rod bearings, is that just the connecting end on the downside of the piston?

 

The connecting rod bearings are the in between the rod and the crank. When those get too much clearance, they make a horrible knocking noise. If your friend doesn't know exactly, call the mechanic that looked at it to get as much correct info as you can so you know what your looking for.